The present invention is directed to the field of paper forming machines, and more particularly to hydrodynamic foils employed in open wire paper formers, such as the fourdrinier wire section.
The fourdrinier wire section or open wire paper former is the oldest and still most widely used paper forming method, and briefly involves the deposition of a fiber suspension onto an endless wire mesh running horizontally. The water drains through the mesh, while the majority of the fibers are retained by the wire. At the end of the table, most of the water has been drained from the sheet, and the sheet is separated from the endless wire and collected in rolls.
One of the techniques for removing the water from the sheet is through the use of a device commonly referred to as a table roll. Initially, table rolls were used because they offer the best way to support the wire with the least resistance to moving it. However, it was shown that the table roll creates a vacuum in the water filled gap, or nip, between the wire and the roll, thus making it a very powerful drainage tool. However, the intense pressure drop encountered by the wire when moving over the roll creates very high vertical acceleration forces and thus may tend to disturb the sheet, form ridges, and spout at high speeds.
Another technique for removing water from the sheet which has been increasing in popularity, is the use of hydrofoils or "foils". Prior art foils are stationary blades held in contact with the wire at the front end and diverging from the wire to form a nip at the trailing end at angles typically varying between one and five degrees. The same hydrodynamic principles inherent in the table roll are also present in the use of foils, the diverging nip being functionally equivalent to the trailing portion of the table roll. However, due to the stationary nature of the foil, the vacuum attained over the foil nip is significantly less than the amount which can be developed on a table roll, and the foil is a less efficient drainage element relative to the table roll for this reason. However, the nip on the foil can be much longer than the effective nip of the table roll, and the number of foils along the table can be varied to thereby offset the drainage capability of the foil relative to the table roll. Also, by varying the foil angle, the magnitude of the drainage can be easily controlled, thereby reducing disturbances during the paper forming process. The lower levels of disturbance available with the use of foils results in better formation, less two-sidedness, better retention, increased speed limit, reduced loss of additives or fines from the wire side of the sheet. Further, the use of the double nip hydrofoil substantially increases the water removal capacity of the Fourdrinier allowing the lowering of headbox consistency.
By adjusting the angle of inclination of the trailing end of the foil, particular operating characteristics can be achieved. For example, if maximum water removal is the only goal, the use of a long blade with an approxmately 2.degree. angle at the beginning of the foil station and the use of a foil with an approximately 1.degree. angle at the end of the foil station will probably be used. However, common practice is to install alternate foils of varying angles along the wire path in order to optimize the rate of water removal without creating excessive sheet disturbance and high loss of fines.
Despite the judicious use of the above described foils having the nip at the trailing end of the foil, improvement of paper formation on the fourdrinier is still desired. Specifically, it has been found that under certain circumstances, the dispersion of flocs initially formed on the wire side cannot be achieved without significant mat disruption and removal of fines from the wire side. Additionally, when attempting to reduce the rate of water removal in order to eliminate excessive sheet disturbance and loss of fines, removal of too little water may result in the phenomenon known as "sealing" of the sheet on the wire, making subsequent water removal more difficult. Additionally, flow streaks from the slice rectifier roll, and uneven edges, are many times problems encountered in the use of prior art machines.